In solvent extraction processes, organic phases consisting of an organic extractant, organic diluent, and optionally organic soluble compounds commonly called "phase-modifiers," are contacted with a metal value loaded aqueous phase in a counter-current, cross-current, or co-current fashion over one or more stages. The organic phase is selected in such a way so as to be immiscible with the aqueous phase. Hydrogen ions from the organic phase exchange with metal ions from the aqueous phase so that the organic phase becomes loaded with metal values while the aqueous phase emerges depleted in these metal values. The pH of the aqueous phase is usually controlled to maintain the efficiency and to adjust the metal selectivity of the exchange process. After the extraction, the metal loaded organic phase is contacted with acid in a counter-current, cross-current, or co-current fashion over one or more stages to transfer metal values to the aqueous solution. The metal values from this aqueous solution can then be recovered by various means, for example, by electrowinning.
Effective nickel and cobalt recovery from dilute acidic leach solutions, such as those from leaching lateritic ores, has been known to be possible by precipitation with hydrogen sulfide. When precipitating with hydrogen sulfide, the resulting mixed nickel/cobalt sulfide precipitate may be further refined by operations that optionally may include solvent extraction. Advantageously, solvent extraction is performed on the leach solution directly to bypass the sulfide precipitation step. This direct solvent extraction route eliminates the costs associated with the difficult production and handling of hydrogen sulfide gas and has the potential to directly produce market products.
Direct solvent extraction processes have been commercially applied to recover copper for many years. With respect to cobalt and nickel, solvent extraction has been limited almost exclusively to the refining of intermediate nickel-cobalt products (Bautista, R., "The Solvent Extraction of Nickel, Cobalt, and their Associated Metals." Extractive Metallurgy of Copper, Nickel and Cobalt, vol. I; Fundamental Aspects, R. Reddy and R. Weizenbach (Editors), The Minerals, Metals, and Materials Society, 1993, pp. 827-852). The only exception is for the direct nickel/cobalt recovery from ammoniacal leach liquors as described, for example, in U.S. Pat. Nos. 3,907,966 and 3,981,968.
Attempts to adapt existing solvent extractants for recovery of nickel and cobalt from solutions obtained as a result of direct acid leaching of ores or concentrates using, for example, sulfuric acid have been largely unsuccessful. One main reason is that these solutions typically contain significant amounts of dissolved manganese, magnesium and/or calcium, and these metals are often extracted together with nickel and cobalt. For example, organophosphorus and carboxylic acid extractants extract cobalt and nickel, but also co-extract, often even preferentially, manganese (and to a lesser extent calcium and magnesium). The co-extraction of these metal ions consumes a significant portion of the extractant loading capacity and does not allow the obtaining of pure strip liquors. This eventually renders the extractant commercially unacceptable. Furthermore, excess aqueous solubility is typically a problem for extractants such as carboxylic acids.
Extractants comprising a mixture of carboxylic acids and non-chelating oximes have demonstrated nickel and cobalt selectivity over manganese, magnesium, and calcium. However, the non-chelating oximes usually have high aqueous solubilities and tend to hydrolyse. Chelating hydroxyoxime extractants, such as ketoximes and salicyl aldoximes, most of them commercially developed for the extraction of copper (II) from sulfuric acid leach solutions, have also demonstrated selectivity for nickel and cobalt (II) over manganese, calcium, and magnesium. However, once loaded into these chelating oximes, cobalt (II) tends to oxidize to cobalt (III), which adversely affects stripping and may degrade the oxime reagent. Furthermore, the rate for nickel extraction using chelating oxime extractants has been reported as being very slow (Szymanowski, J., Hydroxyoximes and Copper Hydrometallurgy, CRC Press, 1993, p. 281). Mixtures of chelating hydroxyoximes with di-nonyl naphthalene sulphonic acid (DNNS) have demonstrated improved nickel extraction, however, the DNNS accelerates the degradation of the oxime (Oliver, A. J., and Ettel, V. A., "LIX 65N and Dowfax 2 AO Interaction in Copper Solvent Extraction and Electrolysis," CIM 14th Annual Conf., Edmonton, August 1975, pp. 383-88).
Brown et al., in U.S. Pat. No. 4,721,605 disclose a method whereby the metals selected from the group consisting of zinc, silver, cadmium, mercury, nickel, cobalt, and copper can be separated from calcium and/or magnesium, present in an aqueous solution, by solvent extraction using dithiophosphinic acids. Furthermore, B. T. Tait reported in "Cobalt-Nickel Separation: The Extraction of Cobalt (II) and Nickel (II) by Cyanex 301, Cyanex 302, and Cyanex 272," Hydrometallurgy, 32 (1993) pp. 365-372, that Cyanex 301 extractant (Cyanex is a trademark for organophosphorus extractants distributed by Cytec Canada Inc.) extracts both nickel and cobalt and may also be used to selectively remove cobalt from nickel-containing solution. However, the difference in the pH values at which 50% of the cobalt and 50% of the nickel is extracted is a relatively small value of only 1.1 units. In this article, Tait also noted the disadvantage of requiring a strong acid to strip cobalt when using Cyanex 301 extractant. Tait also suggested in "The Extraction of Some Base Metal Ions by Cyanex 301, Cyanex 302 and Binary Extractant Mixtures with Aliquat 336," Solv. Extr. Ion Exch., 10(5) (1992) pp. 799-809, that manganese is also extracted by Cyanex 301 at some higher pH values than nickel and cobalt. Sole et al. in "Solvent Extraction Characteristics of Thiosubstituted Organophosphinic Acid Extractants," Hydrometallurgy, 30 (1992) pp. 345-65, illustrate almost no separation between nickel and cobalt for Cyanex 301. Contrary to Tait, Sole et al. have indicated that Cyanex 301 displays a slight preference for nickel over cobalt. Furthermore, Cote and Bauer have described in "Metal Complexes with Organothiophosphorus Ligands and Extraction Phenomena," Reviews in Inorganic Chemistry, Vol. 10, Nos. 1-3, (1989) pp. 121-144, that the class of organothiophosphorus acid extractants can be oxidized to disulphides by Fe (III) present in the solution as well as by Co (III) which may form in the organic phase as a result of oxidation of Co (II) by atmospheric oxygen. Cote and Bauer further noted that the oxidation of Co (II) to Co (III) may be avoided in the presence of oxygen donor reagents in the organic phase such as tri-octyl-phosphine oxide (TOPO), tri-butyl-phosphate (TBP), or octanol (ROH).
None of the foregoing discloses a commercially viable process for the selective recovery of metals such as nickel and/or cobalt against metals such as manganese, calcium and magnesium in acidic solutions. The acid leaching of nickeliferous lateritic ores, for example, generates leach solutions containing nickel and cobalt, often combined with appreciable amounts of impurities such as manganese and magnesium. Therefore, a welcome contribution to the art would be a method for the selective solvent extraction of only nickel and cobalt from aqueous solutions containing these metals as well as manganese, magnesium and the like. After the primary separation of nickel and cobalt from other metals by solvent extraction, additional solvent extractants are often required for the separation of nickel from cobalt. A single extractant capable of separating nickel and cobalt from other metals and separating nickel from cobalt would further contribute to the field of nickel and cobalt recovery.
It is an object of the invention to provide a process to selectively recover nickel and/or cobalt values from acidic aqueous solutions using solvent extraction and avoiding the co-extraction of other metal values, present in the same solution, such as, but not limited to, manganese, calcium, and magnesium.
It is a further object of the invention to provide a simple and economical process consistent with the preceding object.
It is a further object of the invention to provide reusable stripped organic phase consistent with one or both of the preceding objects.
It is a further object of the invention to provide separation of nickel from cobalt by their selective stripping from loaded organic phase consistent with one or more of the preceding objects.
It is a further object of the invention to provide separation of nickel from cobalt by their selective loading into an organic phase consistent with one or more of the preceding objects.